Various facets of the biosynthetic pathway of vitamin B12 are not yet understood. It is proposed to investigate the manner of incorporation of L-threonine into the (R)-1-amino-2-propanol moiety of vitamin B12 by means of various in vitro systems, using enzymes and substrates from different bacterial sources. It is also proposed to obtain an enzyme system to study the mode of amidation of the carboxylic side chains of the corrinoid ring. If time permits, details of a system able to convert ALA to corrinoids will be investigated.